This invention relates to a time interval measuring instrument for measuring a time interval, in particular, a very short time interval, with high accuracy by counting clock pulses.
A method that has heretofore been employed for measuring a time interval involves counting of the number of clock pulses occurring in the time interval to be measured. In this case, the higher the clock pulse frequency is, the greater the measurement accuracy is. But it is impossible to use a clock pulse whose frequency is higher than the resolution of a counter for counting the clock pulses, and a counter capable of counting clock pulses of high frequency is expensive.
To avoid the above defect, there has been employed a method in which the time interval to be measured is converted to a voltage, this voltage is converted again to a time interval longer than the original one, and then a count is taken of the number of clock pulses occurring in the expanded time interval. With this method, as compared with the case of such conversion being not effected, it is possible to increase the measurement accuracy by a multiple ratio of the expanded time interval to the original one if the clock pulse frequencies used are the same. Conversely, if the measurement accuracy required is the same, the clock pulse frequency used can be reduced, and consequently an inexpensive counter can be employed.
In time interval measurements involving such time expansion, however, if a plurality of time intervals to be measured are successively applied in a relatively close relationship, then before measurement of one of the incoming time intervals by conversion is completed, the next successive time interval to be measured occurs, so that these plural time intervals cannot be measured by the same converter. It is also possible to measure the time interval to be measured by converting it to a voltage and further converting the voltage to a digital signal by a method which is different from the method of the type involving counting clock pulses. In this instance, a plurality of time intervals, even if occurring in a relatively close relationship, can be measured by the employment of a high-speed A-D converter. But such an A-D converter is very expensive.
For measuring a plurality of time intervals occurring relatively close together by the method of the type involving counting of clock pulses, one might consider preparing pluralities of voltage-time converters and counters, applying the time intervals converted to voltage to the individual voltage-time converters, respectively, and counting the numbers of clock pulses by the individual counters for the expanded time intervals. With this method, however, the measuring instrument involves parallel connections of pluralities of voltage-time converters, and counters and hence is expensive. In addition, the conversion characteristics of the voltage-time converters are subject to aging and variations due to ambient temperature change and must be housed, for example, in a constant-temperature oven so as to prevent such variations, resulting in the measuring instrument becoming markedly bulky and expensive. Moreover, it is difficult to keep the conversion characteristics of the voltage-time converters constant at all times, so that their measured outputs fluctuate.
For counting high-frequency clock pulses during a relatively long time interval to be measured, the counter to be used is required to have an enormous number of stages, and hence is costly. If the clock pulse frequency is reduced, the number of stages of the counter may be small but the measurement accuracy drops. In view of this, in order to increase the measurement accuracy, there has been employed a method in which the number of clock pulses of a relatively low frequency is counted during the time to be measured, and the time intervals between the start of the time to be measured and the next successive clock pulse, and between the end of the time to be measured and the next successive clock pulse, are measured by using clock pulses of a higher frequency, or these time intervals are expanded and the number of clock pulses of a relatively low frequency is measured during each of the time intervals. Such a measuring method is disclosed, for example, in U.S. Pat. No. 3,133,189, issued May 12, 1964. With this method, however, it is necessary to house a time-voltage converter and a voltage-time converter in a constant-temperature oven so as to protect the measurement from the influence of ambient temperature variations.
It is an object of this invention to provide a time interval measuring instrument which is capable of measuring not only a very short time interval but also a plurality of closely spaced time intervals with high accuracy.
It is another object of this invention to provide a time interval measuring instrument which provides highly accurate measurements of very short time intervals occurring relatively close to each other and which can be constructed at low cost.
It is another object of this invention to provide a time interval measuring instrument which does not employ any constant-temperature oven but enables highly accurate time interval measurements untouched by ambient temperature change.
It is another object of this invention to provide a time interval measuring instrument which enables accurate time interval measurements untouched by nonlinearity of the conversion characteristics of a time-voltage converter and a voltage-time converter.
It is still another object of this invention to provide a time interval measuring instrument which is capable of measuring a relatively long time interval at low cost and with high accuracy.